1. Field of the Invention
The present invention relates to a slurry Si-base composition for use in producing an Si.sub.3 N.sub.4 ceramic, which composition contains water as a dispersion medium. Also, the present invention relates to a dense Si.sub.3 N.sub.4 ceramic of a high strength produced from the above slurry Si-base composition and processes for producing these.
2. Description of the Prior Art
Alcohols have been employed as a dispersion medium for ensuring an effective dispersion of a silicon (Si) powder in the mixing/molding step when a ceramic is produced from the silicon powder as a starting material, as described in Mat. Res. Soc. Symp. Proc., vol. 60, pp. 51-61 (1986). However, from the industrial point of view, the process in which use is made of an alcohol must be provided with expensive explosionproof facilities, which constitute a cause of the increase of product cost.
On the other hand, Japanese Patent Laid-Open Nos. 261662/1991 and 212279/1989 proposed processes in which water not requiring explosionproof facilities is used as a dispersion medium. These processes produce a sintered body of Si.sub.3 N.sub.4 through dispersion of an Si powder in water, addition of an organic binder, a nitriding agent, etc. so as to obtain a slurry, dehydration of the slurry, molding and heating for sintering the molding.
However, the above processes described in Japanese Patent Laid-Open Nos. 261662/1991 and 212279/1989 fail to add a sintering aid required for densification of the resulting sintered body. Therefore, for example, as clearly set forth in Japanese Patent Laid-Open No. 261662/1991, the obtained sintered body of Si.sub.3 N.sub.4 cannot help having a multiplicity of minute pores dispersed therein. Consequently, the flexural strength of the sintered body of Si.sub.3 N.sub.4 is anticipated to be very low. Actually, only a flexural strength as low as 600 MPa or less has been realized.
The reason for the failure to add a sintering aid to the water-base slurry in the above conventional processes would be that the dispersion of Si powder and sintering aid is poor in water as different from that in an alcohol, thereby causing the resultant specimen to have a nonuniform density therein.